The present invention relates to wireless communication technology and more particularly to the technology of controlling transmission power in each frequency band in a wireless communication system of cellular type.
Mobile wireless communication is required to constitute service area spreading as surface in order to communicate moving terminals with a base station, and the cellular system is generally used. In the wireless communication system of cellular system, plural base stations or antennas are distributed dispersedly and areas named cells in which wireless communication can be made are set within the range where radio waves from base stations or antennas are reached, so that the service area of wireless communication spreading as surface is formed. There is a case where the base station uses directional antenna to divide cell angularly and provides plural wave arrival areas named sectors. The cell configuration is generally three-sector configuration in which the cell is divided into three areas. The sector can be regarded as the cell formed by dividing space angularly by utilization of directivity of antenna. In the present invention, hereinafter, there is a case where the cell includes the concept of both of sector and cell.
In the wireless communication system, when a terminal can receive signal from plural base stations or antennas, the terminal is controlled to be connected to the base station having highest signal quality. Further, the wireless communication system has the structure of handover for handing over the base station for connecting the terminal successively with movement of the terminal, so that the wireless communication system can maintain wireless communication even while terminal is being moved. In order to secure connectivity upon movement of terminal between service areas, the service areas formed by base stations or antennas overlap each other in boundaries thereof. Signals transmitted by base stations or antennas are information for terminals connected to base stations or antennas, although the signals are interference for communication of terminal connected to other base stations or antennas. Such interference is disturbance for terminal subjected to the interference and causes deterioration in communication quality and reduction in throughput.
In order to reduce interference between base stations in boundaries of service areas, there is a case where area design is made in which three base stations adjacent to one another utilize different frequencies and three frequencies are allocated to base stations repeatedly within the wireless communication system to avoid interference. It is said to be used in Reuse 3. In contrast, area design that the same frequency is utilized repeatedly is said to be Reuse 1.
In the mobile WiMAX (refer to IEEE 802.16-2004 Part 16: Air Interface for Fixed Broadband Wireless Access System) paragraph 8.4 Wireless MAN-OFDMA PHY), there is known the structure of increasing frequencies capable of being used by base stations while reducing interference among base stations. In this structure, transmission powers of frequency resources are weighted in plural base stations or antennas mutually or frequency resources are selected to thereby share frequency resources to be utilized among plural base stations or antennas, so that occurrence of interference is suppressed and this structure is described as FFR (Fractional Frequency Reuse) in “Mobile WiMAX-Part I: A Technical Overview and Performance Evaluation” WiMAX Forum, March 2006. In this technique, frequency is used in Reuse 3 only in boundary parts of cells among adjacent cells and frequency is used in Reuse 1 in center parts of base stations.
Further, as technique of reduction in interference, there is a method described in JP-A-2009-21787. In this patent document, transmission frame is divided into sub-channel subset use zone and sub-channel use zone in time axis direction and sub-channel subset use zone is divided into plural zones in frequency axis direction, so that connection is allocated to previously decided zone in order of priority to form priority zone to which power necessary to improve signal quality until the signal quality reaches necessary signal quality is allocated.